The present invention relates to a throttle apparatus for an internal combustion engine and, more particularly, to an electronic control throttle device which controls opening and closing a throttle valve by driving an electric driven actuator based on a control signal.
In the electronic control throttle apparatus for controlling a throttle valve of an engine by driving an electric driven actuator (for example, a direct current motor, a stepping motor), a technology known is that an initial opening degree (default opening degree) of the throttle valve in an off state of engine key (in other words, at not energized state of the electric driven actuator) is set to a position larger than its full close position.
Here, the full close position does not mean a position for completely choking the intake air passage. Particularly, in a throttle device performing idling rotating speed control using only a throttle valve without any bypass passage for bypassing the throttle valve, the full close position is defined by classifying into a mechanical full close position and an electrical full close position to be described below.
The mechanical full close position means a minimum opening degree position of a throttle valve determined by a stopper, and the minimum opening degree is set at a position to slightly open the throttle valve from a position to completely choking the intake air passage in order to prevent the throttle valve from sticking. The electrical full close position means a minimum opening degree within a range of opening degrees used for control, and the minimum opening degree is set at an opening position slightly larger than the mechanical full close position in taking the mechanical full close position as the reference by controlling of driving the electric driven actuator (for instance, a position larger than the mechanical full close position by approximately 1°). In the electronic control throttle device, the electrical full close position (the minimum opening degree on the control purpose) does not always agree with an idling opening degree (an opening degree necessary for idling rotating speed control). The reason is that because the opening degree of the throttle valve is feedback controlled based on an idling rotating speed detected signal in order to keep the idling rotating speed to a target rotating speed, and thereby the opening degree can not be determined uniquely.
In regard to a full open position, there are a mechanical full open position determined by a stopper and an electrical full open position of a maximum opening degree on control. Therein, in a case of simply describing a “full close position”, meaning of the word includes the electrical full close position as well as the mechanical full close position. In a normal control, the throttle valve is controlled between the electrical full close position (the minimum opening degree on the control purpose) and the electrical full open position (the maximum opening degree on the control purpose). By doing so, a part of the throttle valve does not hit on the stoppers for determining the mechanical full close position and the mechanical full open position at controlling the throttle valve to the minimum and the maximum opening degrees. Therefore, mechanical fatigue, abrasion and damage of the stoppers and the gear members can be prevented and sticking of the throttle valve to the stopper can be prevented.
A default opening degree (that is, the initial opening degree in an off state of engine key) is set to an opening degree of a position in which the throttle valve is further opened wider than that in the full close position (the mechanical full close position and the electrical full close position)(for example, a position larger than the mechanical full close position by 4 to 13°). One reason why the default opening degree is set is that an air flow rate necessary for combustion of pre-warming-up operation at stating-up of the engine (cold starting-up) is secured without any auxiliary air passage, (an air passage bypassing the throttle valve). During idling operation, as the engine is warmed up, the throttle valve is controlled so as to moved from the default opening degree toward the smaller opening degree. However, the lower limit is the electrical full close position. Another reason why the default opening degree is set is to cope with requirements for securing self-running (limp home) or for securing an intake air flow rate to prevent the engine operation from stopping even if the throttle control system is failed, for preventing the throttle valve from fixing to an inner surface of the throttle body with a viscous substance or ice.
As conventional examples of default opening degree setting mechanisms, various kinds of default opening degree setting mechanisms are proposed in, for example, Japanese Patent Application Laid-Open No. 63-150449, U.S. Pat. No. 4,947,815 and the corresponding patent of Japanese Patent Application Laid-Open No. 2-500677, Japanese Patent Application Laid-Open No. 62-82238 and the corresponding patent of U.S. Pat. No. 4,735,179 by the same applicant of the present invention, Japanese Patent Application Laid-Open No. 10-89096, Japanese Patent Application Laid-Open No. 10-131771 and so on.
There are various types of default opening degree setting mechanisms. For example, one type is that a default opening degree is secured by setting the relationship between forces of a returning spring for acting a force toward a closing direction of the throttle valve and an opposed spring (called as a default spring or an initial opening degree spring) for acting a force toward an opening direction of the throttle valve opposing against the force of the returning spring so that the force of the default spring is larger than the force of the returning spring at the default opening degree position and so that a free end of the default spring is stopped by a default stopper at the default opening degree position when the engine key is switched off (for example, Japanese Patent Application Laid-Open No. 2-500677).
Another type is, as disclosed in Japanese Patent Application Laid-Open No. 1-131771, that a fixing side engaging element to be fixed to a throttle valve shaft (this engaging element may be constructed by a throttle lever, or a gear for transmitting motor power may be used instead of the engaging element) and a moving side engaging element (a linking lever) idly inserted onto the throttle valve shaft and rotatable relative to the throttle valve shaft are provided, and the moving side engaging element and the fixing side engaging element are linked together with a returning spring so as to attract each other, and a force is applied using a default spring onto the moving side engaging element in a direction of opening the throttle valve to engage and rotate the moving side engaging element and the fixing side engaging element (the throttle valve shaft) together opposing against the force of the default spring when the opening degree is within the range smaller than the default opening degree (smaller than the default stopper position), and to rotate only the fixing side engaging element and accordingly the throttle valve shaft opposing against the force of the returning spring and preventing movement of the moving side engaging element by the default stopper when the opening degree is within the range larger than the default opening degree. On the contrary, there is a type that the moving side engaging element and the fixing side engaging element are linked together with the default spring so as to attract each other, and a force is applied using the returning spring onto the moving side engaging element in a direction of closing the throttle valve to engage and rotate the moving side engaging element and the fixing side engaging element (the throttle valve shaft) together opposing against the force of the returning spring when the opening degree is within the range larger than the default opening degree, and to rotate only the fixing side engaging element (the throttle valve shaft) opposing against the force of the default spring and preventing movement of the moving side engaging element by the default stopper when the opening degree is within the range smaller than the default opening degree.
The electronic control throttle device can more accurately perform air flow rate control suitable for operation of an internal combustion engine than a mechanical throttle device in which an amount of stepping-in of the accelerator pedal is transmitted to a throttle valve shaft through an accelerator wire. However, since the electronic control throttle device has the electric drive actuator and the default opening degree setting mechanism, number of the parts is increased and accordingly it is important how to make the throttle body small in size, light in weight and simple in structure and how to simplify the wiring (wire harness).
Further, the electronic control throttle device controls the idling rotating speed by controlling opening degree of the throttle valve, but has the following point to be improved.
In a case where idling rotating speed control is performed with the throttle valve in the electronic control throttle device, an opening degree larger than the mechanical full close position by a certain angle (for example, 5 to 1°) is secured at least as the minimum opening degree on the control purpose. Since a gap (sometime called as a shaft support gap) between the throttle valve shaft and a shaft inserting through hole provided in a wall of the throttle body which guides the throttle valve shaft to a bearing practically serves as a part of the intake air passage and the air flow rate (leak air flow rate) flowing through the shaft support gap can not be controlled, the minimum opening degree on the control purpose is set with taking it into consideration that the leak flow rate flows into the internal combustion engine.
However, according to the conventional set value of the minimum opening degree for the control purpose (the electrical full close position), when the throttle valve is closed from the maximum opening degree for the control purpose (the electrical full open position) toward the minimum opening degree (the electrical full close position, in the idling state), a magnitude of overshoot becomes larger than the minimum opening degree in the closing direction (the overshoot is approximately 1.5° at maximum) because the driving force of the motor (the electric drive actuator) is decreased at high temperature or at low temperature (that is, the torque of the motor is reduced at high temperature due to increase in the resistance of the motor, and the torque of the motor is reduced at low temperature due to decease in the battery voltage). As a result, as shown by a solid line {circle around (1)} in FIG. 17, the throttle valve hits on the stopper at the mechanical full close position (the diagonally shaded area in FIG. 17 indicates a state that movement of the throttle valve is blocked by the full close stopper.), and over-current flows in the motor likely to cause an erroneous fail-safe diagnosis (an erroneous diagnosis judging of occurrence of failure in the motor from the over current) or decrease in the lifetime of the motor.